1. Field of the Invention
The present invention relates to a two-dimensional image display device and, more particularly, relates to improvement of video display devices such as a video projector, a television receiver, a liquid crystal panel and the like.
2. Description of the Related Art
In recent years, an image projection device has been widespread as a two-dimensional image display device using a high-pressure mercury discharge lamp as a light source. In this device, light emitted from the high-pressure mercury discharge lamp is separated into red light (long wavelength light), green light (intermediate wavelength light), and blue light (short wavelength light) by using a wavelength selection mirror, and the respective color lights are individually modulated by a liquid crystal panel and then multiplexed by a dichroic prism, whereby a color image is projected on a screen using a projection lens. However, the spectrum emitted from the lamp covers the entire visible area, and the spectrum of the light divided by the wavelength selection mirror has a relatively wide spectrum width exceeding 100 nm. Therefore, bright pure colors cannot be displayed. Accordingly, a laser display capable of brighter color representation has attracted attention. The laser display employs three kinds of laser light sources corresponding to red, blue, and green, and it is constructed as shown in FIG. 9.
With reference to FIG. 9, reference numeral 200 denotes a laser display including laser light sources 1a˜1c for emitting laser lights corresponding to three colors of R, G, and B, diffusers 6a˜6c for diffusing light, and an optical system for irradiating the diffusers 6a˜6c with the laser lights outputted from the laser light sources 1a˜1c. The laser display 200 further includes diffuser wobbling means 13a˜13c for wobbling the diffusers 6a˜6c, and spatial light modulators 7a˜7c for modulating the lights that are emitted from the respective laser light sources 1a˜1c and diffused by the diffuser wobbling means 13a˜13c. The laser display 200 further includes a dichroic prism 9 for multiplexing the lights that pass through the spatial light modulators 7a˜7c, and a projection lens 10 for enlarging and projecting the light multiplexed by the dichroic prism 9 onto a screen 11.
The laser light source 1a is a red laser light source that emits red laser light. An optical system corresponding to this red laser light source 1a includes a beam expander 2a for expanding the light emitted from the laser light source 1a, and a light integrator 3a for making the in-plane intensity distribution of the light expanded by the beam expander 2a uniform. Further, this optical system includes a condenser lens 12a for condensing the light emitted from the light integrator 3a, a mirror 15a for reflecting the light condensed by the condenser lens 12a, and a field lens 8a for converting the light reflected from the mirror 15a into convergent light and irradiating the diffuser 6a with the convergent light.
The laser light source 1b is a green laser light source that emits green laser light. An optical system corresponding to this green laser light source 1b includes a beam expander 2b for expanding the light emitted from the laser light source 1b, and a light integrator 3b for making the cross-section intensity distribution of the light expanded by the beam expander 2b uniform. Further, this optical system includes a condenser lens 12b for condensing the light outputted from the light integrator 3b, and a field lens 8b for converting the light condensed by the condenser lens 12b into convergent light and irradiating the diffuser 6b with the convergent light.
The laser light source 1c is a blue laser light source that emits blue laser light. An optical system corresponding to this blue laser light source 1c includes a beam expander 2c for expanding the light emitted from the laser light source 1c, and a light integrator 3c for making the cross-section intensity distribution of the light expanded by the beam expander 2c uniform. Further, this optical system includes a condenser lens 12c for condensing the light outputted from the light integrator 3c, a mirror 15c for reflecting the light condensed by the condenser lens 12c, and a field lens 8c for converting the reflected light from the mirror 15c into convergent light and irradiating the diffuser 6c with the convergent light.
The lights emitted from the red, green, and blue laser light sources 1a, 1b, and 1c are expanded by the beam expanders 2a, 2b, and 2c, and the expanded lights pass through the light integrators 3a, 3b, and 3c and the condenser lenses 12a, 12b, and 12c, respectively. The optical paths of the red light and the blue light are bent at 90 degrees by the mirrors 15a and 15c. The red, green, and blue lights irradiate the spatial light modulators 7a, 7b, and 7c through the field lenses 8a, 8b, and 8c and the diffusers 6a, 6b, and 6c, respectively. Meanwhile, the lights pass through the light integrators 3a, 3b, and 3c, whereby the luminance distributions thereof on the spatial light modulators 7a, 7b, and 7c become uniform. The lights individually modulated by the spatial light modulators 7a, 7b, and 7c are multiplexed by the dichroic prism 9, and the multiplexed light is enlarged and projected by the projection lens 10 to be focused on the screen 11. At this time, since the laser light is highly coherent, speckle noise is imposed on the image projected on the screen. In order to avoid the speckle noise, the diffusers 6a, 6b, and 6c are wobbled by the diffuser wobbling means 13a˜13c, whereby the speckle noise is temporally averaged to be reduced.
The greatest characteristic of the device shown in FIG. 9 is as follows. That is, since each of the lights emitted from the laser light sources has a very narrow emission spectrum of 5 nm or less, the color range that can be expressed by mixing the lights becomes very broad. FIG. 7 is a chromaticity diagram (1931 CIE chromaticity diagram) expressing the color range. In FIG. 7, a range shown by a triangle with Δ marks as apexes is a color range of a video signal defined by the NTSC standard, and a range shown by a triangle with O marks as apexes is a color range obtained when a red light source having a center wavelength of 633 nm, a green light source having a center wavelength of 532 nm, and a blue light source having a center wavelength of 457 nm are employed. As is evident from the chromaticity diagram, the color range of the laser display (the region inside the three O marks) is larger than the color range that can be expressed by the NTSC signal (the region inside the three Δ marks) excluding a small portion of the blue region, resulting in brighter color representation.
By the way, especially in the red and blue regions, since a difference in the widths of the color regions significantly affects the sharpness and realism of the image, a red light source of a longer wavelength and a blue light source of a shorter wavelength are required. However, when employing a red light source of a longer wavelength and a blue light source of a shorter wavelength, visibility of human eyes is drastically degraded, larger output power light sources are needed.
As described above, it has been considered that, in order to realize a practical two-dimensional image display device, the respective light sources must be set at optimum wavelengths, in view of the trade-off relationship between the width of the color region and the required light source output. For example, according to Japanese Published Patent Application No. Hei. 10-293268 (Pages 3-7, FIGS. 2-6), it is desirable to use a red light source having a wavelength of 635 nm or less which does not cause significant reduction in visibility, and a blue light source having a wavelength of 455 nm or less which also does not cause significant reduction in visibility.
A major problem in realizing the above-mentioned laser display is luminance efficiencies of the respective laser light sources. The conventional laser display has employed, as a light source, a vapor laser such as a helium-neon laser or a Krypton laser, or a combination of a YAG (Yttrium Aluminum Garnet) solid laser and a non-linear optical element, which performs wavelength conversion. These light sources have relatively low emission efficiencies, and their sizes and power consumptions are undesirably increased in order to realize bright wide-screen display. Therefore, the scale of the whole device is undesirably increased, which prevents realization of practical laser displays.
The present invention is made to solve the above-described problems and has for its object to provide a two-dimensional image display device which can solve the problems of increased sizes and power consumptions of light sources, and which can emit pure white color.